usually heay period but very light this month. what could it be? stress are medical?

Usually heavy period for 7 days til i use up a whole box of tampons every month. This month its very light and i only see it when i wipe. Before i would have sex and start bleeding while having sex are afterwards but it would go off day later. Could it be stress are medical? I had my tubes tied two years ago and never had problems besides heavier period. Please let me know something